1. Field of the Invention
The present invention relates to novel drug targeting agents. More particularly, the invention relates to drug agents having a unique targetor which affords the agents access to mitochondria via the acylcarnitine translocase system. These novel drug targeting agents are useful in the treatment of cancer, arthritis, skin, metabolic (i.e. diabetes), immunological (i.e. AIDS, tissue rejection) and neurodegenerative disorders (i.e. dementia, Alzheimer's disease). The agents also have useful diagnostic and monitoring applications.
2. Description of the Prior Art
Transport of exogenous substances such as therapeutic or diagnostic agents across cell membranes into mitochondria is highly selective and tightly controlled by two separate membrane barriers. Access to mitochondria is achieved through the acylcarnitine translocase system. L-Carnitine (.gamma.-amino-(R)-.beta.-hydroxybutyric acid trimethylbetaine), acting in concert with mitochondrial acyltransferases, CPT.sub.o and CPT.sub.i, and an enzyme known as the carnitine acylcarnitine translocase, facilitates transport of fatty acids through the inner membrane of the mitochondria and into the matrix where the energy yielding fatty acid .beta.-oxidation, as well as ketone body synthesis takes place.
Consequently, L-camnitine is an essential natural compound and is found in both plant and animal tissues. Though there is a propensity in vertebrates for L-carnitine to concentrate in heart and muscle tissue, both of which depend heavily on fatty acid oxidation for the production of energy, carnitine also concentrates 10 to 100 fold higher in the liver, kidney, brain, testes and epididymis than in plasma. Thus, a variety of diseases can be targeted with a caritine delivery system.
While it has been known for some time that carlitine mediates the energy requirements of a cell, this property has not been taken advantage of in therapeutic applications. Indeed, only as recently as 1991 has interest in this natural product as a targeting ligand of pharmaceutical agents surfaced. U.S. Pat. Nos. 4,742,081, 4,866,040 and 5,008,288 teach the use of carnitine-coupled pharmaceuticals to deliver those pharmaceuticals to cardiac and skeletal muscle. Specifically, these patents contemplate coupling carnitine to protease inhibitors and cardioactive compounds such as pepstatin, leucylarginal, procainamide, quinidine, and propranalol. Not contemplated however, is the use of carnitine as a targeting ligand of antitumor, antiemitic, or antiartritis agents.
Neoplastic disorders constitute a major health problem in the world today. Few antineoplastic agents have the dual beneficial properties of both efficacy and reduced toxicity The vast majority of antineoplastic agents currently in use are generally both relatively non-tumor specific, as well as toxic to the individual being treated. For example, typical toxicities associated with antitumor therapeutic agents include immune suppression, bone marrow depression, alopecia, and a host of other unwanted side effects. The key in identifying beneficial antineoplastic agents is in isolating agents which are capable of inhibiting neoplastic growth without adversely affecting normal cell growth.
Cisplatin has been known since 1845 when it was first named Peyrone's salt or Peyrone's chloride and used primarily in the development of coordination theory. However, it was not known to have cytotoxic effects until the serendipitous work of Barnett Rosenberg in 1962. By the end of the 1970's cisplatin had become a key drug in the treatment of certain germ cell cancers. In the past 20 years, cisplatin's antitumor efficacy has expanded to include ovarian and testicular tumors, oropharyngeal carcinoma, bronchogenic carcinoma, cervical carcinoma, melanoma, lymphoma, bladder carcinoma, neuroblastoma and others. The major obstacles to the efficacy of cisplatin in the treatment of cancer are poor water solubility, resistance to the drug and toxicity. Common side effects of cisplatin include nephrotoxicity (kidney damage), ototoxicity (damage to the nerves or organs involved in hearing and balance), myelosuppression (bone marrow suppression) resulting in leukopenia (depletion of leukocytes from the blood), neutropenia (depletion of neutrophils), or thrombocytopenia (depletion of platelets), neurotoxicity (damage to peripheral nerves), and a marked emesis (nausea and vomiting). Side effects occasionally include serum electrolyte imbalances, ocular toxicity (damage to the optic neurons), vascular toxicities, and cardiac abnormalities. Anaphylactic shock may also occur within minutes of administration in patients that are allergic to platinum (generally through occupational exposure).
Since the initial report on the anti-neoplastic properties of cisplatin, thousands of analogs have been made in an attempt to alleviate these adverse properties of cisplatin. Although roughly thirteen of these analogs have been tested in clinical trials, only one analog, carboplatin, has been a definite improvement over cisplatin. Carboplatin has a similar anti-tumor profile a, cisplatin. However, carboplatin causes less emesis, and is less toxic to the eighth cranial nerve, the peripheral nerves, and renal tubules. The decreased nephrotoxicity allows carboplatin to be administered on an outpatient basis. Bone marrow suppression is the dose-limiting side effect of carboplatin. The structures of cisplatin and carboplatin are as follows: ##STR1##
In spite of these disadvantages, the unparalleled clinical importance of cisplatin and carboplatin in the treatment of many types of solid tumors justifies the effort to find a therapeutically more palatable analog.
The cisplatin analogs that have been studied to date are described in Raymond B. Weiss & Michaele C. Christian, New Cisplatin Analogues in Development, Drugs, 46(3): 360-377 (1993); Maxwell Gordon & Sandra Hollander, Review of Platinum Anticancer Compounds, J. Med. 24(4-5); 209-265 (1993); and L. Steven Hollis, New Approaches to the Design of Platinum Antitumor Agents, Platinum and Other Metal Coordinating Compounds in Cancer Chemotherapy, 115-125 (Stephen B. Howell, ed. 1991), all of which are incorporated herein by reference.
Cisplatin and carboplatin are known to exhibit their anti-cancer activities by interfering with DNA replication. Though not yet fully elucidated, the currently accepted mechanism by which cisplatin interacts with chromosomal (nuclear) DNA is as follows: In die presence of DNA, exchange of a labile leaving group leads to preferential, though not exclusive, binding at the N(7) position of a guanine base. Binding also occurs at the N(7) position of an adenine (A) base, but not as strongly. Coordination to two proximally disposed guanines (most common) or to a guanine or adenine (less common) base, respectively, affords Pt-GpG and Pt-ApG type inter- and intrastrand cross-links. It is postulated that the slower hydrolysis of the malonato (1,1,-cyclobutanedicarboxylic acid) leaving group in carboplatin, as compared with the highly labile chlorine ions in cisplatin, as well as it imparting increased aqueous solubility accounts for carboplatin's decreased toxicity. Intrastrand cross-linking is thought to be the critical lesion because it is believed that this type of bond causes only minor disruption to the DNA conformation. Minor changes are less likely to activate the DNA repair enzymes which have the DNA under constant surveillance. In contrast, interstrand, crossstrand and DNA-protein cross-links frequently activate repair enzymes which then remove the lesion. Resistance to further assault by agents of similar mechanism is rapidly acquired and thus, acquired resistance to cisplatin and similar drugs is encountered.
The correlation between the chemical structure of cisplatin analogs and their activities has been defined. Essentially, antitumor activity is best expressed when the platinum complex falls into the formula, cis-A.sub.2 Pt.sup.II X.sub.2 or cis-A.sub.2 Pt.sup.IV X.sub.2 Y.sub.2 structures. "A" is a cis-oriented organic amine ligand, preferably a primary amine or ammonia. A secondary amine is considerably less effective and a tertiary amine or non-hydrogen donating nitrogen will generally render the complex inactive. If A.sub.2 represents a bidentate ligand (a ligand that is chelated to a metal ion by means of 2 donor atoms), it must be cis-coordinated and sterically nonobtrusive. "X" is required to be at least one easily exchangeable ligand such as chloride or a carboxylate group, and "Y" can represent two additional trans-oriented exchangeable polar groups such as RO.sup.- or chloride. The active configuration of platinum is square planar and in the oxidation state of II. If introduced in the IV oxidation state the complex is cisplatin like inactive unless reduced in vivo to the II oxidation state by iron II or ascorbic acid, for example.
While the overall antitumor activity of cisplatin has not been significantly improved upon, replacing the chlorides with the less labile, but still exchangeable, malonato group, as in carboplatin, enhances aqueous solubility and considerably reduces dose-limiting nephrotoxicity and severe emetic properties.
In spite of many attempts, a single strategy that specifically addresses the crucial solubility, toxicity, and cell penetration problems has hitherto not been adequately addressed.
The approach utilized in the present invention involves targeting an agent for entry into the mitochondria Mitochondrial DNA (mtDNA) is considerably less complex than chromosomal DNA, yet it is a fully functional DNA that is equally vital to the cell. It is circular in structure and encodes for the two mitochondrial ribosome RNAs and 22 mitochondrial transfer RNAs that remain in the mitochondria to produce all 13 of the human proteins associated with mitochondrial function. Since the mitochondria modulate the energy requirements of a cell, tumor or otherwise, a disfunction in mtDNA replication would consequently kill the cell. In addition, mitochondria are intracellular components and, therefore, a method that targets an agent for entry into the mitochondria also potentially renders accessible other intracellular material such as chromosomal DNA in the process.